mongodb-source: Changed properties username and password as optional.

A Mongodb server can be configured without authentication, for example for testing purposes.
diff --git a/mongodb-source.kamelet.yaml b/mongodb-source.kamelet.yaml
index 3738e3e..995df36 100644
--- a/mongodb-source.kamelet.yaml
+++ b/mongodb-source.kamelet.yaml
@@ -22,8 +22,6 @@
     required:
       - hosts
       - collection
-      - password
-      - username
       - database
     type: object
     properties:
@@ -74,8 +72,8 @@
       parameters:
         hosts: "{{hosts}}"
         collection: "{{collection}}"
-        password: "{{password}}"
-        username: "{{username}}"
+        password: "{{?password}}"
+        username: "{{?username}}"
         database: "{{database}}"
         persistentTailTracking: "{{persistentTailTracking}}"
         tailTrackIncreasingField: "{{?tailTrackIncreasingField}}"